The present invention generally relates to a DC bus short circuit compliant power generation system, and more particularly, to a DC bus short circuit compliant power generation system having an internal bus that does not collapse when the main DC bus is shorted.
Power generation systems (PGS) play a significant role in the modern aerospace/military industry. Recently, this has been particularly true in the area of more electric architecture (MEA) for aircraft and spacecraft. The commercial aircraft business is moving toward a no-bleed air environmental control system (ECS), variable frequency (VF) power distribution systems, and electrical actuation. The next-generation Boeing airplane (the Boeing 787 platform) and the Airbus airplane will most likely use MEA. Some military aircraft already utilize MEA for primary and secondary flight control among other functions.
These new aerospace trends have significantly increased power generation needs. This has led to increased operating voltages to reduce system losses, weight, and volume, and a new set of power and electromagnetic interference (EMI) requirements has been created to satisfy both quality and performance. The overall result has been a significant increase in the installed electric power, creating challenges in accommodating this equipment in the new platforms. Therefore, overall system performance improvement and power density increases are necessary for the new generation hardware to satisfy MEA. The power generation systems' cost is an additional driver that needs improvement to make the new platforms affordable.
The high performance power generation system (HPPGS) applicable to MEA is required to satisfy a quite complex set of requirements. Generation is the main function of such a system, providing conversion of the mechanical power supplied by the prime mover to conditioned electrical power supplied to the distribution bus. Generation system rating is typically defined as continuous power at 100% load. Increasing the load to 150% for a limited time may be required. The percentage of increase and time required for overloading varies from application to application. Self-start is a motoring operation that provides engine startup. Systems use power supplied by an electric distribution bus, an electric auxiliary source, or a battery and create a predetermined startup profile for the prime mover. This function may last from several seconds to several minutes. An efficient startup function is typically challenging because of limited power availability. Motoring is a function that provides continuous motoring operation using an external electric power source for maintenance purposes. Different speeds may be required. Finally, short circuit current function is required when an external short circuit fault occurs at the DC distribution bus. The protection system uses it for clearing the fault. This current is slightly higher than the maximum operating current and is required for several seconds.
The synchronous permanent magnet machine (PMM) presents a very competitive design that outperforms other electric machines in most applications when weight and size are critical. However, the rotor flux in a typical PMM is fixed and cannot be controlled or disengaged when a short circuit is initiated. Unlike other machines where the excitation of the rotor flux can be controlled and even disabled quickly, a PMM continues to generate electromotive force (EMF) until the rotor stops rotating. Therefore, the PMM presents a hazard in some applications, leading to its limited use, particularly in the aerospace industry.
The high reactance permanent magnet machine (HRPMM) was conceived to internally limit the phase current magnitude, should it become shorted, to levels capable of being sustained either indefinitely, within the thermal limits of the system, or until the rotor speed can be reduced to zero.
This feature of the HRPMM makes the PMM-based PGS much safer. However there are applications where the fault currents must be discontinued instantly. This is possible only if electric machines with external rotor flux excitation are used. The most popular machine representatives are wound-field (WF), switch-reluctance machines (SRM), and induction machines (IM). Recent high-speed technological advancements make the IM a very strong candidate for this application. However a special provision is required when a DC bus short circuit is experienced and the generation system is supposed to deliver short circuit current to clear the fault protection circuit. The external source for the IM excitation is shorted. Therefore, an alternative is required to supply excitation for the IM rotor flux.
FIG. 1 shows a conventional simplified architecture of a power generation system 10 using an induction machine (IM) 12. The power generation system 10 may include a three-phase IM 12, a three-phase bridge 14 (D1 through D6 and Q1 through Q6), a DC link capacitor C1, an EMI filter 16, a main contactor 18, a battery contactor 20, and a control module 22.
The IM 12 may interface mechanically with a prime mover via direct coupling or a gearbox (not shown). The main function of the IM 12 is to convert electrical power to mechanical power when used for motoring and self-start modes of operation, and to convert mechanical power to electrical power when used as a generator. Various types of IM's 12 can be used. The most popular design is a three-phase winding 24 accommodated in the slots of a laminated tooth type stator (not shown).
The three-phase bridge 14 (D1 through D6 and Q1 through Q6) may operate as an inverter during motoring and self-start operation. The bridge 14 may create the required voltage at the IM terminals resulting in three-phase equally spaced currents required for machine control. In generating mode, the bridge 14 may operate as an exciter for the machine rotor flux while it acts as an active rectifier.
The DC link capacitor C1 may provide a low impedance source for the bridge 14 operating as a voltage source inverter. During generation, C1 may provide filtering of the rectified voltage.
The purpose of the EMI filter 16 is to provide adequate filtering for radiated and conducted emission to meet the respective requirements for electromagnetic compliance EMC. Also, the EMI filter 16 may protect the internal control circuits from external interference.
The main contactor 18 connects and disconnects the main bus (not shown) of the power generation system 10 when required for normal operation or emergency.
The battery contactor 20 connects and disconnects the battery bus (not shown) of the power generation system 10 when required for normal operation or emergency.
The control module 22 may receive control signals and send status signals to an external host computer (not shown). The control module 22 may also receive measured internal signals from the system, which is required for control and protection purposes such as currents, voltages, temperatures and status of contactors. The control system 22 may generate control signals to the actively controlled semiconductor and electromagnetic devices.
This power topology may provide generation, self-start and motoring modes of operation by using an appropriate control system as described above. However, supplying short circuit current to the DC bus when an external or internal fault occurs across the DC bus cannot be performed. The reason is lack of a supply source required for the bridge to excite IM.
As can be seen, there is a need for an apparatus for an improved family of IM-based power generation systems that can supply current to the bus in case of short circuit failure.